| Metal materials play an important role in modern industrial manufacturing.With the widespread use of metal products,there are many safety problems occurring due to the unqualified performance and quality of metal products.For the active service of metal products,it is necessary to inspect them.It is especially important to conduct nondestructive,convenient,qualitative and quantitative research and analysis on the performance of metal products in service.As a physical quantity that is closely related to the performance of metal materials,conductivity can be used to characterize the heat treatment,fatigue,hardness and other performances of metal materials.Eddy current method is used to the measurement on the conductivity of metal materials due to its non-destructive and fast speed.Based on the eddy current detection technology,the conductivity of non-ferromagnetic metal is studied by COMSOL.Moreover,a conductivity meter for non-ferromagnetic metal is developed with differential probe and detection circuit.Finally,the performance of the developed instrument is evaluated on detection accuracy and error.In this paper,a finite element model of eddy current is established based on the electromagnetic field finite element theory,and the effects of lift-off,electrical conductivity,and excitation frequency on the impedance of coils are simulated and analyzed.Combined with the conductivity simulation and system configuration,the relationship between the conductivity of the test piece and the system output is studied,and the relationship between the conductivity and the differential probe signal is obtained.The systematic error due to the measurement method is studied,and the measurement method on conductivity is proposed.Then,based on the eddy current theory and impedance analysis method,the conductivity measurement hardware system is designed.The function and principle of each hardware circuit are described,the performance of each part of the circuit is presented,and the packaging of the entire hardware system and probe is completed.The device can detect small changes in impedance and process the results in the main control chip.Then,the software program of conductivity measurement has been programed,and the control of the circuit by the main control chip has been achieved,and the calculation of the conductivity of the test piece is completed.The interface of the conductivity instrument has been designed to realize the communication between the serial port screen and the single-chip computer and the human-computer interaction function.Optimized the data collection during the detection and improved the accuracy of the detection results.Finally,a experiment of conductivity measurement was designed to evaluate the performance of instrument.The conductivity measurement experiment sample was prepared,and the standard conductivity of the experiment sample was obtained using a commercial eddy current conductivity meter.And use the developed conductivity instrument to verify the relationship between conductivity and impedance,and analyze the error and accuracy of the instrument.The results show that the phase of differential output signal is not affected by the amplifier multiplier,which reduces the performance requirements of the hardware system,and has a higher resolution at low conductivity.The accuracy of the calibrated conductivity instrument is 0.06 MS/m;when conductivity is not greater than 20 MS/m,the absolute error of the measurement result is not more than ±0.25 MS/m;when conductivity is greater than 20 MS/m,The relative error of the measurement result does not exceed ±5%.This paper contains 63 figures,13 tables and 87 references. |
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